The present invention relates to processes for transmitting service signals of a digital radio beam. It also relates to the transmitters and receivers enabling the use of such a process in radio beams.
In order to use a radio beam it is necessary to transmit on the latter not only the signals such as speech signals for which it is intended, but also a certain number of supplementary signals, such as remote monitoring or service channels. These supplementary signals are called service signals.
When the modulation transmitted by the radio beams is of an analog nature, these service signals are usually in the form of analog tones.
In digital radio beams in which the carrier is directly modulated according to a certain number of states selected in a digital manner on the basis of digital input signals and able to represent both data and speech, it is both simpler and more advantageous to use these service signals in the form of digital signals.
To transmit these service signals, it is firstly possible to multiplex them with the main signal to be transmitted and then transmit the thus obtained frame, demultiplexing it on reception to separate the thus transmitted service signals. However, this has the disadvantage of increasing the power of the radio beam and the need to use a multiplexer and a demultiplexer on reception, which are costly and large pieces of equipment.
It is also known to transmit these service signals without increasing the power by superimposing an analog modulation, which can be of phase, frequency or amplitude, on the main modulation provided by the train of digital signals. In order that such a modulation superimposition does not disturb the main transmission, it is necessary to bring about a limitation to a relatively low service signal capacity.